Antigen-mediated stimulation of hydrolysis of inositol phospholipids in RBL-2H3 cells is dependent on the extent and rate of aggregation of the plasma membrane receptors for IgE. The hydrolysis is associated with an increase in concentration of free Ca2+ (Ca2+)i, an increase in cytosolic pH and histamine secretion. Under optimal conditions for stimulation, the rate of generation of inositol polyphosphates, increase in (Ca2+)i and increase in cytosol pH reach a maximum at, respectively 15 to 30, 180 and 300 to 500 seconds following addition of antigen. Interestingly, the pH increase becomes most apparent once (Ca2+)i begins to decline. This pH signal appears to be generated through activation of protein kinase C and as a consequence of activation of the Na+/H+ antiport. Accordingly the increase in pH can be reproduced by sole addition of PMA an activator of protein kinase C, but neither PMA nor antigen provoke a pH signal in Na+-free or neutral (pH 7.0) buffers. The pH signal explains previously unexplained effects of PMA which were attributed to a "cryptic signal" in antigen stimulated cells (J.Cell Biol. 105:1129, 1987). Studies with intact, permeabilized and broken cell preparations and variously labeled inositol phosphates have also provided an almost complete map of the pathways of metabolism of the inositol phosphates in RBL-2H3 cells and point to an interesting interconversion of inositol 1,4,5-trisphosphate and inositol 1,3,4,5- tetrakisphosphate.